Above the fuel assemblies in the vessel of a pressurized water nuclear reactor, there is a set of internal equipment, often termed "upper internals", which comprises an upper support plate and a lower plate connected by spacer columns. Between the two plates there are also guide tubes for control clusters which are suspended from the upper support plate, and which rest on the lower plate on which they are held by resilient pins. The lower plate, which also constitutes the upper plate of the core proper, has centering pins projecting from its lower face which engage in corresponding parts of the upper end of the fuel assemblies to assure that they are held in position.
During periods when the reactor is being reloaded, it is necessary to remove the upper internal equipment so as to have access to the fuel. This equipment is radioactively contaminated, and must be stored under water in the swimming pool during fuel reloading or transfer operations. A support stand is provided for this, constituted by a circular flange which rests on the bottom of the swimming pool on four feet. To allow a sufficient height of water to be provided above the upper plate and to assure radiological protection without excessively increasing the volume of the swimming pool, the support stand is located in a deeper part of the swimming pool, forming a pit of relatively small surface. In the storage position, the upper support plate of the upper internals rests on the circular flange of this stand, and the lower plate is then at a very small distance from the bottom of the deeper pit of the swimming pool.
It is particularly advantageous to use the time during which the reactor is shut down for reloading for simultaneously monitoring the upper internals in the storage position on the stand, and monitoring in particular the state of the lower face of the lower plate so as to check the state of the plate proper, the centering pieces for the assemblies, or resilient pins for the guide tubes of the control clusters. But these monitoring operations are difficult, particularly because of the small amount of space available between the lower plate and the bottom of the pit in the swimming pool, and also because of the smallness of this deeper pit. Lastly, so as not to prolong reloading operations, and consequently the time the reactor is shut down, it is necessary that the reloading machine provided to move above the swimming pool not be used too long for auxiliary operations.
The present invention provides for a self-contained machine adapted to work under the lower plate, and not necessarily requiring use of the loading machine.